1. Field of the Invention
This invention relates to methods for dewaxing petroleum lube feedstocks by a process involving the synergistic steps of catalytically dewaxing such stocks with a catalyst of the structure of ZSM-5 followed by solvent dewaxing, without recycle of the treated feed.
2. Discussion of the Prior Art
Processes for dewaxing petroleum distillates are known. Essentially, two preferred processes for dewaxing of petroleum feedstocks exist, solvent dewaxing and catalytic dewaxing.
Catalytic dewaxing is described, for example, in the Oil and Gas Journal, Jan. 6, 1975, pages 69-73. A number of patents have also described catalytic dewaxing processes. It is known that a high pour point oil may be catalytically dewaxed to a lower pour point over catalysts of the structure of ZSM-5. Such catalysts selectively crack long chain normal paraffins, slightly branched iso-paraffins and long chain cyclo-paraffins. For example, U.S. Pat. No. Re. 28,398 describes a process for catalytic dewaxing a feedstock with a catalyst of the structure of ZSM-5 having a hydrogenation/dehydrogenation component. The process for hydrodewaxing a gas oil with a catalyst of the structure of ZSM-5 is also described in U.S. Pat. No. 3,956,102.
Current technology for dewaxing petroleum feedstocks having elevated pour points involves the use of a dewaxing reactor having trickle beds, whereby gas (primarily hydrogen) and a liquid feedstock concurrently flow downward over a bed of solid catalyst. The three-phase trickle bed concept makes use of an intimate mixing between gas and liquid phases while in contact with a solid catalyst in order to facilitate dewaxing. Performance level of the process is gauged by the length of time during which the process is producing products of specification, as well as the minimum temperature required to obtain an acceptable product.
Typically, the dewaxing reactor is operated at a start of cycle temperature of 540.degree. to about 580.degree. F. (232.degree.-304.degree. C.). The operating temperature is increased by about 2.degree. to about 10.degree. F. per day--depending on feed, catalyst and space velocity--compensating for decreased catalyst activity to produce a lube of predetermined pour point. Temperatures are increased to an end-of-cycle temperature of between 655.degree. and about 695.degree. F. (346.degree.-368.degree. C.), usually about 675.degree. F. (357.degree. C.). At the end of cycle, usually about 10 days, the reactor must be shut down to regenerate the catalyst.
Dewaxing catalyst regeneration is expensive and is usually accomplished by high temperature H.sub.2 regeneration conducted between 900.degree. to 980.degree. F. (482.degree.-526.degree. C.). However, it is known that with each subsequent regeneration of catalyst, the catalyst loses activity. This loss of activity is caused by residual amounts of nitrogen, sulfur and oxygen left on the catalyst. After a predetermined level of deactivation, oxygen regeneration is employed to burn the residue off the catalyst and achieve activity resembling that of fresh catalyst. Although oxygen regeneration restores catalyst activity, such treatments are expensive, and the high temperature required for regeneration can result in catalyst sintering. Catalyst regeneration is described in more detail in U.S. Pat. Nos. 3,904,510; 3,986,982; and 3,418,256.
For very waxy feeds, catalytic dewaxing may require multiple dewaxing steps to attain a desired product pour point. A feedstock, such as Minas bright stock, with a boiling point greater than 1000.degree. F. (537.degree. C.) and a wax content of greater than 20 wt %, as measured by ASTM, has to be processed three times before the product's target pour point can be obtained.
It is also known, as disclosed above, to subject a waxy lube to solvent dewaxing. Solvent dewaxing generally consists of mixing a solvent, such as methylethylketone, with a waxy feed and chilling the mixture to precipitate the wax. The mixture is filtered or centrifuged to remove the precipitated wax and the solvent is recovered for reuse. When a feedstock contains a large amount of waxy component, that is a feedstock having a wax content of greater than 20 wt %, as measured by ASTM, multiple solvent dewaxing processing steps are necessary to dewax the feedstock. Where a specific pour point is to be reached for a particular waxy feed, as many as seven solvent dewaxings may be required to attain the target pour point with minimal lube product losses. Of course, less dewaxings will be required for less waxy stocks. Attempting to reach a specific pour point by passing a very waxy crude through a single solvent dewaxing step would entrain most of the lube product in the wax, drastically reducing the yield of the desired product.
It is also known to produce a high quality lube base stock oil by subjecting a waxy crude oil fraction to solvent dewaxing, followed by catalytic dewaxing. Such a process is described in U.S. Pat. Nos. 4,181,598 to Gillespie and 3,755,138 to Chen et al. However, these processes are designed to remove the higher boiling point waxes from the feed without converting them to lighter products. Solvent dewaxing, followed by catalytic dewaxing, is not an economically feasible process for dewaxing waxy feedstocks. Solvent dewaxing, followed by catalytic dewaxing is not operational for waxy crudes, such as Minas and some Chinese crudes. A solvent dewaxer would only be able to remove a given quantity of wax, and the subsequent wax loading on the catalytic dewaxing unit would be so great that the catalyst would be rendered inactive within hours, instead of the commercial cycle of ten days. Standard lube specification product pour points of less than 20.degree. F. (-7.degree. C.) could not be obtained for crudes such as Minas.
In view of the disadvantages disclosed above, it is an object of this invention to provide a process for producing a lube oil having a targeted pour point.
Another object of this invention is to provide a simple and economic process to obtain a high quality lube base stock while operating a dewaxing reactor at much longer cycle lengths.
Another object of this invention is to provide a process for producing a high quality lube base stock, wherein the high boiling point waxes are subjected to catalytic dewaxing to remove a percentage of these waxes, producing a partially reduced pour point stock and then subjecting the reduced stock to solvent dewaxing.